Determinations of tear film osmolarity were performed to evaluate their usefulness in diagnosing keratoconjunctivitis sicca (KCS) and to evaluate the possible role of elevated tear osmolarity in this disorder's pathogenesis. Tear samples were obtained using a new technique that virtually eliminates the problems of reflex tearing and sample evaporation. The tear osmolarity of 36 samples obtained from 31 normals eyes averaged 302 +/- 6.3 (SD) mOsm/liter; 38 samples obtained from 30 KCS eyes averaged 343 +/- 32.3 (SD) mOsm/liter. The sensitivity of a single measurement was 94.7% and the specificity was 93.7%. Tear samples taken on separate occasions from one normal subject ranged between 295 and 309 mOsm/liter; those obtained from a KCS patient ranged between 312 and 424 mOsm/liter. Hypersmolarity of the tear film in KCS may play an important role in inducing the disease seen in the cornea and conjunctiva.